The invention pertains to the processing of a stream of liquid chlorine containing nitrogen trichloride from a chlorine production process, for example a chloralkali production process.
In the industrial production of chlorine, a small amount of by-product nitrogen trichloride (NCl3) is produced. In a chloralkali production process the amount formed is proportional to the amount of ammonia present in the salt fed to the process. Nitrogen trichloride follows the product chlorine leaving the chloralkali cell house.
Nitrogen trichloride is an unstable compound that detonates when it reaches a critical concentration, reported to be about 13 wt %, though it is believed that nitrogen trichloride decomposes to create dangerous conditions at concentrations as low as 3 wt %. However, a critical mass of nitrogen trichloride is also required before it is considered capable of damaging equipment. According to a report of the Chlorine Institute Inc. a typical chlorine vessel with a wall thickness of one-half inch can be fractured with as little as 1.5 gm/cm2 liquid film of pure nitrogen trichloride. As a compound with a significantly lower vapor pressure than chlorine, it can concentrate if liquid chlorine, containing nitrogen trichloride, is allowed to evaporate. Nitrogen trichloride is thought to be the cause of explosions and fatalities in chloralkali production facilities.
Chlorine product is typically supplied as a liquid, but the end-user normally evaporates the liquid chlorine prior to use. Depending on how the chlorine is evaporated, this can lead to an increase in nitrogen trichloride concentration. A critical part of the chlorine production process is therefore to keep the nitrogen trichloride concentration low in the final chlorine product, typically only a few parts per million, to allow the end-user to safely evaporate the liquid chlorine. In the chloralkali process, nitrogen trichloride is often removed from the product chlorine through an absorption step, for example in a chlorine scrubber, prior to chlorine compression and liquefaction. In the scrubbing step, nitrogen trichloride is absorbed into fresh, clean product chlorine and pushed down the scrubber and into a holding tank, referred to as the nitrogen trichloride decomposer, containing carbon tetrachloride or sometimes chloroform. In the decomposer, the solvent is maintained at a temperature above the boiling point of the chlorine. When the liquid chlorine contacts the warm solvent, it flashes back into the chlorine scrubber while nitrogen trichloride is absorbed by the solvent. Conditions in the decomposer are selected so that nitrogen trichloride slowly and safely decomposes. In time, tars and other impurities build up in the solvent, and the solvent must be periodically replaced, generating a waste stream that must be disposed of.
For reasons of both regulatory constraints and product quality (i.e. to reduce organic content in the final product chlorine), it is desirable to avoid the use of solvents such as carbon tetrachloride and chloroform in the chlorine production train.
A method of disposing of nitrogen trichloride without using carbon tetrachloride or chloroform solvents is described in U.S. Pat. No. 3,568,409, Ferguson et al., in which gas chlorine from the drying tower is contacted with hydrochloric acid upstream of the compression and liquefaction steps. However, the process produces an acidic waste stream that must be disposed of or used elsewhere.
In the method described in U.S. Pat. No. 3,568,409, chlorine is vaporized and recycled back to process but this is done only after removing or destroying the nitrogen trichloride content. In the currently-practiced industrial process, chlorine is also vaporized and recycled back to process but only after nitrogen trichloride is absorbed into carbon tetrachloride or chloroform or other appropriate organic solvents. The vaporization of chlorine is an important step of most methods proposed for the removal of nitrogen trichloride from the final chlorine product of a chloralkali production facility.
Although it would be desirable to be able to vaporize the liquid chlorine stream containing a high concentration of nitrogen trichloride from the chlorine scrubber to avoid the use of a decomposer and organic solvents or the use of other chemicals that generate waste streams that must be handled, the chlorine vaporizers currently used in industry have shortcomings in vaporizing such streams. Industrial chlorine vaporizers are generally non-horizontal units, such as vertical bayonet style units, or horizontal vaporizer units, such as kettle reboiler style units. For convenience in the following discussion, non-horizontal units with a positive slope are referred to as “vertical” units, meaning units with an angle from the horizontal from 0.1 to 90 degrees. These horizontal and vertical chlorine vaporizers can be of two types, namely: pool boiling vaporizers and plug-flow vaporizers. In a pool boiling vaporizer, such as a vertical bayonet or kettle reboiler style vaporizer, liquid chlorine is evaporated out of a main body of liquid chlorine. Compounds with lower vapor pressure than chlorine concentrate in the main body of liquid chlorine as chlorine is evaporated. In an upward flow vaporizer, such as a Hooker-style vaporizer or tube vaporizer, liquid chlorine is evaporated as it moves through the vaporizer. In an upward flow vaporizer, depending on the developed flow regime, compounds with lower vapor pressure than chlorine can locally concentrate within the unit, as described below. Neither type of vaporizer is normally used to vaporize liquid chlorine containing a high concentration of nitrogen trichloride, because of the danger of concentrating it. Euro Chlor recommends that the concentration of nitrogen trichloride in liquid chlorine in “reboilers” be maintained below 1000 ppm to avoid excessive concentration. There is a balance between the tendency to concentrate and the tendency for nitrogen trichloride to decompose which is complex and not completely understood.
In a conventional vertical upward flow vaporizer, chlorine flows the length of the unit through three distinct regime zones. In the first zone, liquid chlorine is heated to its boiling point. In the second zone chlorine is evaporated and in the third zone the resulting chlorine gas is superheated. It is within the second zone, i.e. the boiling zone, of a vertical upward flow vaporizer that nitrogen trichloride can concentrate dangerously. However, reflux of liquid from the boiling zone to the preheat zone can also cause concentration.
In the boiling zone of a conventional vertical upward flow vaporizer, three flow regimes are commonly encountered. The first flow regime occurs at the beginning of boiling when little vapor has yet formed and a “churn” flow regime develops. In the churn flow regime, vapor and liquid randomly mix and back-mix. Once enough vapor is generated, an “annular” flow regime develops, where vapor flows through the center of the heat transfer chamber (e.g., tube) pushing the liquid forward and against the heat transfer wall. Finally, sufficient vapor is generated to take the system into a “mist” flow regime, where the liquid is broken down into small droplets that randomly mix with the vapor while it moves forward through the boiling zone. It is in the boiling zone where a churn flow regime is present that nitrogen trichloride can concentrate and mass build-up. In this zone, liquid chlorine can back-mix and local “pool” boiling can develop.
Once the annular flow regime develops within the boiling zone, the liquid is forced forward by the vapor generated, and back-mixing, and therefore nitrogen trichloride accumulation, is minimal.
Back-mixing is also minimal in the mist flow regime. As liquid flows forward through the annular and mist flow regimes, liquid chlorine is evaporated faster than the dissolved nitrogen trichloride. However, the mass of nitrogen trichloride in the liquid is decreasing and not accumulating.
In a conventional vertical upward flow chlorine vaporizer it is difficult to ensure that nitrogen trichloride concentrations do not increase substantially in the zone where the liquid and churn flow regimes are present, and before the annular flow regime is achieved. As a result, this type of vaporizer is not typically used to handle liquid chlorine streams rich in nitrogen trichloride.
In a horizontal or downward slope plug-flow vaporizer, the opportunity of evaporating the liquid chlorine without back-mixing (i.e., without locally concentrating nitrogen trichloride) is substantially increased relative to a vertical upward flow unit. In these units gravity forces the liquid chlorine to flow forward with more and more of the volume of the unit taken by the generated vapor. If properly designed, the boiling churn zone can be avoided altogether. However, the concern with these types of units is that they can be accidentally flooded, for example, by exceeding their vaporization capacity (i.e., feeding too much liquid chlorine). Once flooded, evaporating the liquid chlorine leads to pool boiling rather than plug-flow vaporization. This causes nitrogen trichloride to concentrate, which can lead to an unsafe condition. On the other hand, a vertical upward flow vaporizer flooded with liquid chlorine is quickly drained back to the feeding tank as soon as enough vapor chlorine is generated again, avoiding pool boiling conditions.
In summary, vertical upward, horizontal, and downward flow vaporizers all have shortcomings in dealing with liquid chlorine streams rich in nitrogen trichloride. There is a need in the industry for an improved method of vaporizing chlorine streams containing a high concentration of nitrogen trichloride.
It would also be desirable to provide a process for disposing of the nitrogen trichloride removed from the chlorine stream without using organic solvents such as carbon tetrachloride or chloroform, or other liquid chemicals.